Ceramics and ceramic composites are being used in increasing proportions for various structural applications. One such use is in the fabrication of components for internal combustion engines. Ceramics alone are generally considered to be brittle and, thus, reinforcing fibers or whiskers are often added to produce ceramic composities. Some strengthing and toughening can also be achieved in certain zirconium-containing ceramics through a transformation in zirconium of a normally tetragonal phase to the monoclinic phase caused by the application of a tensile stress. This type of toughening is known as "transformation toughening" and the degree thereof is a function of the composition of the ceramic since the composition affects the ease of transforming the tetragonal phase, i.e., lowering the applied stress required to cause the transformation results in greater toughening effects.
Tetragonal zirconia (ZrO.sub.2) is one of the ceramics that has potential for many structural applications. This material has been shown to be toughened through the transformation process. A number of stabilizing oxides, such as MgO, CaO, CeO.sub.2 and Y.sub.2 O.sub.3 have been used to produce tetragonal zirconia. These oxides are often mechanically mixed with the zirconia powders which can result in compositional inhomogenity of the stabilizer and thus adversely affect any toughening.
It is therefore an object of this invention to provide an improved composition, and a method for making the same, for the transformation toughening of ceramics.
Another object of the invention is to provide a ceramic alloy exhibiting high fracture toughness.
Another object of the present invention is to provide a crystal structure that is substantially all tetragonal phase but that will transform, at least in part, to monoclinic phase when stressed.
A further object of this invention is to provide a method and composition which permits tailoring the toughness of a ceramic body to meet a particular use requirement.
It is also an object of the invention to provide a ternary ceramic alloy of Zr-Ce-Hf oxides having improved and selectable toughness for use in many applications.
These and other objects of the present invention will become apparent upon consideration of the detailed description which follows.